1. Field of the Invention
The present invention relates to a battery charge/discharge control method for an electric vehicle, more specifically to a method of controlling charge or discharge of a battery for the electric vehicle to prevent the battery from being overcharged or over-discharged due to difference in performance among battery cells for constituting the car battery.
2. Information Disclosure of the Related Art
In general, a battery mounted on an electric vehicle is of secondary battery which can be charged and discharged repeatedly. Therefore, when the remaining battery capacity decreases, it is necessary to take an appropriate control or management action before being over-discharged, for instance by charging the battery under restriction of charging current.
Various technique related to the battery charge/discharge control method has been so far proposed. For instance, Japanese Patent Application Laid-Open (Kokai) No. 60-245402 discloses such a battery charge/discharge control method that when any one of the battery open voltage and the battery voltage during vehicle travel drops below a predetermined voltage, respectively, an alarm is generated to inform the driver of a decrease in battery capacity, and in addition the battery capacity and the battery life can be both displayed by detecting the time required until the battery voltage reaches a constant voltage level when charged. By the way, in the electric vehicles, since a relatively high voltage is required as a power source for driving the vehicle, the battery used as the primary power source is usually formed by a combination battery such that a plurality of unit batteries each composed of a plurality of cells are combined with each other. In the case of a lead battery, for instance, a unit battery of nominal 12 V (.apprxeq.2.1 V.times.6) is composed of six cells of 2.1 V, and a battery of nominal 336 V (=12 V.times.28) can be obtained by connecting 28 unit batteries in series. Therefore, the total number of this battery cells is as large as 168 (=6.times.28).
As a result, in the conventional battery charge/discharge control technique of detecting the terminal voltage of the whole battery cells to control the charge or discharge of the battery, since the partial battery cells tend to be overcharged or over-discharged due to difference in performance among the cells during the charge and discharge operation, there exists a problem in that the battery performance deteriorates or damaged, with the result that the battery life is shortened. In addition, once a difference among the individual cells begins to occur due to over-discharge, the battery performance deteriorates sharply with the lapse of time.